For the 1969 season, with Ford and Mercury becoming much
larger contenders, Richard Petty left Plymouth's program
to race for Ford on a single year contract. Plymouth,
seeing the success of the Dodge Charger Daytona, decided
to create a much simlar aerodynamic model. Thus came
the Plymouth Road Runner Superbird for the 1970 season.

The Superbird was basically the Road Runner with:

19-in steel nose with fiberglass popup lights

Fender-top scoops (for tire clearance)

Higher, more slanted rear trunk spoiler

While looking similar, the Superbird and the Daytona did
not use the same added sheetmetal. The Superbird had a
longer nose extension and a more slanted trunk spoiler.
The Superbird won more races than the Daytona, even with
less race entries under its belt. Like the Daytona, the
Superbird would be a one season show, as NASCAR made the
winged cars tune down to a 305-ci max engine in 1971.

Production figures (1920 total):

1162 with the 440-4

665 with the 440 6-pack

93 with the 426 Hemi

NASCAR had a rule change from a 1969 to 1970, instead
of a 500 car production requirement they had to produce
one car per dealer (thus the 1920 number).